"Removable" or "releasable" HMPSA's are those in which the adhesive is permanently adhered to at least one substrate such as paper stock or a polyolefin film layer and then removably attached to a surface in order to secure an article for a period of time. Examples of articles utilizing removable HMPSA's include tapes, tags and labels and adhesive bandages. Feminine napkins, adult incontinent pads as well as some hospital pads and drapes also typically utilize removable HMPSA's commonly described as "garment attachment" or "positioning" adhesives. During manufacturing of such articles, the HMPSA is typically slot-coated directly onto a paper stock or polyolefin film substrate and then covered with a release liner. Alternatively, the adhesive may be transfer coated by first applying the adhesive to the release liner and subsequently contacting the adhesive to the substrate layer, sandwiching the hot melt PSA between the substrate and the release paper.
There are a multitude of patents pertaining to adhesive compositions for tapes, labels, adhesive bandages and the like. Alper et al., EP 525 251 A1, Collins et al., U.S. Pat. No. 4,136,699 and Raykovitz et al., U.S. Pat. No. 4,704,110 specifically relate to hot melt PSA positioning adhesives for feminine napkins. However, such patents are typically directed toward continuous slot-coating of the HMPSA.
EPO 794 202 A1 published Oct. 9, 1997 is directed to aliphatic petroleum-based resins, a method of controlling the softening point and molecular weight of such a resin, and HMPSAs containing such resins. The hot melt adhesive composition generally consists of S-I-S polymers containing 0 to 30 parts per weight of styrene.
In recent years, manufacturers have attempted to replace the continuous slot-coat method with various discontinuous methods, such as engraved roll coating and pattern coating methods, otherwise referred to as screen-printing. There are several advantages in utilizing these types of discontinuous coating techniques including aesthetic advantages due to the variety of designs as well as the ability to strategically place adhesive in precise locations. The primary advantage methods such as engraved roll coating and pattern coating offer is the ability to vastly reduce adhesive consumption since the surface area per mass of adhesive is greatly increased. However, this increase in surface area in combination with the adhesive being applied in small individual deposits rather than a continuous layer creates adhesive transfer concerns, particularly when the substrate coated is a polyolefin film. It is theorized that in order for the adhesive to have the proper balance of properties, the adhesion to the substrate must be greater than the cohesive strength of the hot melt PSA. Concurrently the cohesive strength of the adhesive must be greater than the adhesion to the surface to which the adhesive is removably attached. Adhesives that possess these qualities in a slot coat application typically exhibit adhesive transfer when engraved roll coated or pattern coated, particularly at high removal rates. Furthermore, since directly applying the adhesive to the substrate is preferred for these application methods to enhance adhesion, HMPSA's are needed that are low in viscosity at low application temperatures in combination with the proper balance of adhesion and cohesion. The applicants have found that certain novel hot melt pressure sensitive adhesive compositions meet these requirements.